This section provides background information related to the present disclosure which is not necessarily prior art.
FIG. 1 is a view illustrating one example of a conventional III-nitride semiconductor light emitting device. The III-nitride semiconductor light emitting device includes a substrate 100, a buffer layer 200 grown on the substrate 100, an n-type nitride semiconductor layer 300 grown on the buffer layer 200, an active layer 400 grown on the n-type nitride semiconductor layer 300, a p-type nitride semiconductor layer 500 grown on the active layer 400, a p-side electrode 600 formed on the p-type nitride semiconductor layer 500, a p-side bonding pad 700 formed on the p-side electrode 600, an n-side electrode 800 formed on the n-type nitride semiconductor layer exposed by mesa-etching the p-type nitride semiconductor layer 500 and the active layer 400, and a protective film 900.
In the case of the substrate 100, a GaN substrate can be used as a homo-substrate, and a sapphire substrate, a SiC substrate or a Si substrate can be used as a hetero-substrate. However, any type of substrate that can grow a nitride semiconductor layer thereon can be employed. In the case that the SiC substrate is used, the n-side electrode 800 can be formed on the side of the SiC substrate.
The nitride semiconductor layers epitaxially grown on the substrate 100 are grown usually by metal organic chemical vapor deposition (MOCVD).
The buffer layer 200 serves to overcome differences in lattice constant and thermal expansion coefficient between the hetero-substrate 100 and the nitride semiconductor layers. U.S. Pat. No. 5,122,845 mentions a technique of growing an AlN buffer layer with a thickness of 100 to 500 Å on a sapphire substrate at 380 to 800° C. In addition, U.S. Pat. No. 5,290,393 mentions a technique of growing an Al(x)Ga(1-x)N (0≦x<1) buffer layer with a thickness of 10 to 5000 Å on a sapphire substrate at 200 to 900° C. Moreover, PCT Publication No. WO/05/053042 mentions a technique of growing a SiC buffer layer (seed layer) at 600 to 990° C., and growing an In(x)Ga(1-x)N (0<x≦1) thereon. Preferably, it is provided with an undoped GaN layer with a thickness of 1 to several μm on the AlN buffer layer, Al(x)Ga(1-x)N (0≦x<1) buffer layer or SiC/In(x)Ga(1-x)N (0<x≦1) layer.
In the n-type nitride semiconductor layer 300, at least the n-side electrode 800 formed region (n-type contact layer) is doped with a dopant. Preferably, the n-type contact layer is made of GaN and doped with Si. U.S. Pat. No. 5,733,796 mentions a technique of doping an n-type contact layer at a target doping concentration by adjusting the mixture ratio of Si and other source materials.
The active layer 400 generates light quanta (light) by recombination of electrons and holes. Normally, the active layer 400 contains In(x)Ga(1-x)N (0<x≦1) and has single or multi-quantum well layers. PCT Publication No. WO/02/021121 mentions a technique of doping some portions of a plurality of quantum well layers and barrier layers.
The p-type nitride semiconductor layer 500 is doped with an appropriate dopant such as Mg, and has p-type conductivity by an activation process. U.S. Pat. No. 5,247,533 mentions a technique of activating a p-type nitride semiconductor layer by electron beam irradiation. Moreover, U.S. Pat. No. 5,306,662 mentions a technique of activating a p-type nitride semiconductor layer by annealing over 400° C. PCT Publication No. WO/05/022655 mentions a technique of endowing a p-type nitride semiconductor layer with p-type conductivity without an activation process, by using ammonia and a hydrazine-based source material together as a nitrogen precursor for growing the p-type nitride semiconductor layer.
The p-side electrode 600 is provided to facilitate current supply to the p-type nitride semiconductor layer 500. U.S. Pat. No. 5,563,422 mentions a technique associated with a light transmitting electrode composed of Ni and Au and formed almost on the entire surface of the p-type nitride semiconductor layer 500 and in ohmic-contact with the p-type nitride semiconductor layer 500. In addition, U.S. Pat. No. 6,515,306 mentions a technique of forming an n-type superlattice layer on a p-type nitride semiconductor layer, and forming a light transmitting electrode made of ITO thereon.
Meanwhile, the light transmitting electrode 600 can be formed thick not to transmit but to reflect light toward the substrate 100. This technique is called a flip chip technique. U.S. Pat. No. 6,194,743 mentions a technique associated with an electrode structure including an Ag layer with a thickness over 20 nm, a diffusion barrier layer covering the Ag layer, and a bonding layer containing Au and Al, and covering the diffusion barrier layer.
The p-side bonding pad 700 and the n-side electrode 800 are provided for current supply and external wire bonding. U.S. Pat. No. 5,563,422 mentions a technique of forming an n-side electrode with Ti and Al.
The protection film 900 can be made of SiO2, and may be omitted.
In the meantime, the n-type nitride semiconductor layer 300 or the p-type nitride semiconductor layer 500 can be constructed as single or plural layers. Recently, a technology of manufacturing vertical light emitting devices is introduced by separating the substrate 100 from the nitride semiconductor layers using laser technique or wet etching.
FIG. 2 is a view illustrating one example of an electrode structure described in U.S. Pat. No. 5,563,422. A p-side bonding pad 700 and an n-side electrode 800 are positioned at corner portions of a light emitting device in a diagonal direction. The p-side bonding pad 700 and the n-side electrode 800 are positioned in the farthest portions in the light emitting device to thereby improve current spreading.
FIG. 3 is a view illustrating one example of an electrode structure described in U.S. Pat. No. 6,307,218. According to a tendency toward light emitting devices with large size, finger electrodes 710 and 810 are provided between a p-side bonding pad 700 and an n-side electrode 800 at regular intervals, thereby improving current spreading.